Why 104 capacitor between motor's terminals?

[niemand]

The motor in the attached photo is from an electric toothbrush. I have two such toothbrush. The older generation does not have this 104 capacitor, but the newer one does. What's the purpose?

[Whales]

That's a brushed DC motor.  The contact between the brushes and the armature won't be perfect.  Depending on the motor design and load: you may see sparks between the two during operation.

Perhaps the cap reduces the amount of arcing that occurs after a spark is made?  ie gives current an alternative path to travel when the commutator and brushes lose contact transiently.  That could potentially reduce armature wear, EMI emissions and ozone generation (which might damage plastics nearby?).

I might be completely wrong   You definitely don't want a cap that's too big if you're PWMing the voltage going to the motor.

[Haenk]

it's an easy way to reduce (RF) emissions
pretty much all brushed motors have those installed

[Berni]

This is almost always for EMI reasons.

The capacitor makes for a quick short path for the motors commutator switching noise to close the circuit around to itself rather than use the wires going to the motor as an antenna, radiating it as interference. Without it a DC motor is essentially a spark gap RF transmitter.

[SeanB]

You might also find a pair of 103 ceramic capacitors, soldered between the terminals and the metal body of the motor. They do exactly the same, providing a path for RF energy coupled from the rotor to the case to return, so that you reduce EMI off the motor. On universal motors you commonly find this in a small block, with mains on 2 terminals before the motor, and a third connection to the frame. They normally use the 2 field coils to do double duty as RF chokes as well, one each side of the commutator, so the electrical noise of the brushes is confined to the motor itself and does not escape down the power cord.

[niemand]

You might also find a pair of 103 ceramic capacitors, soldered between the terminals and the metal body of the motor. They do exactly the same, providing a path for RF energy coupled from the rotor to the case to return, so that you reduce EMI off the motor. On universal motors you commonly find this in a small block, with mains on 2 terminals before the motor, and a third connection to the frame. They normally use the 2 field coils to do double duty as RF chokes as well, one each side of the commutator, so the electrical noise of the brushes is confined to the motor itself and does not escape down the power cord.

How would a capacitor, which has two leads, connect to three points (each of the terminals and also the metal body of the motor)?

[TimFox]

Read the statement:  "a pair of 103 ceramic capacitors".
Easy to connect two two-lead parts to three terminals.

[MikeK]

Dealing with Motor Noise:

https://www.pololu.com/docs/0J15/9

[TimFox]

Besides EMI issues, a capacitor across the commutator is similar to one across switch or relay contacts to reduce wear on the contact.